Fluid Milk Processing & Preservation

Relationship between thermal resistance parameter ($z$-value) and temperature coefficient ($Q_{10}$):

• $613.23$ represents an arithmetic transposition error.
• $31.32$ results from miscalculating the exponent as $1.5$ instead of $2.5$.
• The thermal reaction quotient $Q_{10}$ is defined as the factor by which the thermal inactivation rate increases with a $10^\circ\text{C}$ increase in temperature. The mathematical relationship is: $Q_{10} = 10^{10/z}$. Given $z = 4^\circ\text{C}$, $Q_{10} = 10^{10/4} = 10^{2.5} = 10^2 \times 10^{0.5} = 100 \times \sqrt{10} \approx 100 \times 3.162277 = 316.23$.
• $61.23$ is an incorrect calculation.

Factors affecting microbial thermal resistance parameters:

• Increasing D-value would mean microorganisms become more heat resistant, which contradicts the lethal synergistic effect of acid.
• The D-value (decimal reduction time) is the time required at a given temperature to destroy $90\%$ (1 log cycle) of the microbial population. High acidity (lower pH below neutrality) sensitizes vegetative cells and bacterial endospores to thermal destruction, significantly decreasing the heating time required for decimal destruction, thereby reducing the D-value.
• Z-value represents the temperature change required to change D by a factor of 10 and is largely characteristic of the organism's thermal activation energy rather than being directly controlled by acidity in this manner.
• Z-value changes are minor compared to the direct reduction in D-value.

Characteristics and dependencies of decimal reduction time ($D$-value):

• Initial microbial load affects total sterilizing time required ($F$), but the D-value itself represents the time for a 1-log ($90\%$) fraction reduction and is independent of initial microbial population.
• Heating duration is the variable during processing, not a parameter governing the intrinsic D-value constant.
• The D-value is an intrinsic kinetic property that depends directly on the heating temperature (as well as specific organism type and medium composition). According to Bigelow's thermal death time model, higher temperatures exponentially accelerate death kinetics and decrease the D-value.
• Moderate hydrostatic pressure during thermal processing does not dictate thermal D-values in conventional systems.